Longitudinal bone growth occurs at the growth plate, a thin layer of cartilage which consists of three principal zones: the resting zone, the proliferative zone, and the hypertrophic zone. With age, growth plate chondrocyte proliferation slows down, causing longitudinal bone growth to slow and eventually stop. Our previous studies suggest that proliferation slows because stem-like cells, located in the resting zone of the growth plate, have a finite proliferative capacity which is gradually exhausted. Similar replicative senescence occurs when many types of animal cells are placed in primary cell culture, an effect known as the Hayflick phenomenon. However, we found that the number of population doublings of rabbit resting zone chondrocytes in culture did not depend on the age of the animal from which the cells were harvested, suggesting that the mechanisms limiting replicative capacity of growth plate chondrocytes in vivo are distinct from those responsible for limiting replication in vitro. Replicative senescence may be mediated in part by epigenetic changes, including loss of genomic DNA methylation. Growth plate chondrocytes exposed to a demethylating agent in vitro, undergo terminal differentiation. Furthermore, disruption of the SNF2-like gene, PASG, which is required for normal maintainance of DNA methylation, results in growth retardation and premature aging. To investigate this possible mechanism, we measured the overall level of genomic DNA methylation in growth plate cartilage from rabbits of different ages. We found that growth plate senescence is associated with a loss of DNA methylation in resting zone chondrocytes. A similar loss of methylation with age was observed in the proliferative and hypertrophic zone chondrocytes which are thought to be progeny of the resting zone chondrocytes. However, within each age, there was no significant difference in the level of DNA methylation between the different zones of the growth plate. Therefore, loss of methylation appears to occur specifically during replication of resting zone chondrocytes but not during the more rapid proliferation of proliferative zone chondrocytes. This finding suggests that loss of methylation might be involved in the temporal limits that cause chondrocyte replication to slow with age but not the spatial limits that cause chondrocyte proliferation to slow as the cells descend farther down the chondrocyte columns.